This specification discloses a catalytic air cleaner that can purify the air in an enclosed area, such as in an aircraft or a spacecraft, or in other environments.
The present invention represents an improvement over U.S. Pat. No. 4,911,894, which describes in detail the theory of operation of a spirally-wound catalytic air cleaner. This specification hereby incorporates by reference the disclosure of the above-cited patent. That theory applies also to the catalytic air cleaner of the present invention.
The catalytic air cleaner of U.S. Pat. No. 4,911,894 comprises a double spiral formed from two strips of metal foil wound together to form two flow channels, one leading into the core of the spiral, and the other leading out. A combustion catalyst coats the strips of foil. The incoming air stream receives heat by heat exchange with the outgoing air stream, and also from a heater located in the core of the spiral. Catalytic combustion occurs on the surfaces of the strips.
While the catalytic air cleaner of the above-cited patent works well in theory, practical problems arise in building it. These problems have manifested themselves only after the filing date of the above-cited patent. The following paragraphs summarize the problems encountered in making the patented catalytic air cleaner.
In one method of making the air cleaner, one begins with the two strips of metal foil that form the spirals in the finished product. One lays these strips out on the floor, and attaches a strip of spacer along each edge of a strip, using sodium silicate solution as the adhesive. The insulation known as MIN-K super insulation, available from Manville Corp., can serve as the spacer. This insulation has a thermal conductivity as low as that of still air, and thus only half the conductivity of good commercial insulation. When one winds the two strips of foil together to form a spiral, the strips of spacer hold adjacent turns of the spiral apart, and define the side walls of the spiral passages. One seals the end faces of the spiral with the silicone rubber sold under the name RTV-60, available from the General Electric Company. The rubber takes the form of a viscous liquid which one can apply with a brush. The rubber vulcanizes at room temperature (hence the name "RTV", for "room temperature vulcanization") and joins the turns of the spiral and the strips of spacer into a rigid structure.
This silicone rubber cannot withstand a steady state temperature higher than about 260.degree. C., which represents the approximate upper limit for all silicones. But the temperature of the core heater can reach 600.degree. C., which causes the temperature on the face of the spiral nearest the core to exceed 260.degree. C. Insulation of the faces of the spiral makes such elevated temperatures especially likely. On the other hand, if one does not insulate the faces of the spiral, the heat loss becomes prohibitive. The MIN-K super insulation might not prevent overheating on the face even if the insulation had zero conductivity, because the metal foil forming the spiral will itself conduct heat to the face.
In a variation of the above-described construction, one uses ceramic cement to seal the turns of the spiral close to the core. This variation did not succeed because the cement cracked and developed leaks.
Another method of making the patented catalytic air cleaner involves vacuum brazing. In this method, one seeks to attach end closures to the spiral before coating the spiral with catalyst. Thus, the foil defining the turns of the spiral would initially comprise bare metal. One must do the brazing before applying the catalyst for two reasons. First, the high temperature of the vacuum brazing would deactivate the catalyst. Secondly, the activated alumina in the coating would evolve occluded gases and make it impossible to generate the high vacuum needed for brazing.
Before performing the brazing, one winds the metal strips on a spacer of graphite felt. Graphite does not evolve gas at high temperature and high vacuum. Then one brazes wire screens, instead of solid metal sheets, over the ends of the spiral. One can burn out the graphite spacer by holding the brazed spiral in a furnace at about 800.degree. C. Next, one applies the catalyst in the following manner. One dips the spiral into a slurry of activated alumina, and spins off the excess slurry by spinning the spiral about an axis perpendicular to the axis of the spiral. The excess slurry passes through the screen. One applies several coats of alumina in this manner, drying and calcining each coat. Then, one applies the platinum metal catalyst, in a water solution, in the same way. Finally, one seals the screen by painting it with ceramic cement.
In the brazing step, the edge of the foil does not make a continuous line seal with the screen, but instead it makes an interrupted seal with the wires of the screen. In this method, one hopes that sealing the screen with ceramic cement will form a continuous seal with the foil. In practice, this has not happened, so that air leaks between the turns of the spiral, and some of the inlet air never reaches the core.
In summary, neither of the above-described methods of construction has proved practical. Constructing a spirally-wound catalytic air cleaner, where the turns of the spiral bear catalyst before attachment of the end closures, has proved more difficult than anticipated.
The present invention solves the above-described problems by providing a more practical construction for the catalytic air cleaner. In the present invention, the turns of the spiral do not have a catalyst coating at all, and all of the catalytic combustion occurs at the core of the spiral. Thus, it becomes possible to seal the ends of the spiral without deactivating a catalyst by exposing it to high temperatures.